Microscale temperature and SAR measurements in cell monolayer models exposed to millimeter waves

Abstract

Due to shallow penetration of millimeter waves (MMW) and convection in liquid medium surrounding cells, the problem of accurate assessment of local MMW heating in in vitro experiments remains unsolved. Conventional dosimetric MMW techniques, such as infrared imaging or fiber optic (FO) sensors, face several inherent limits. Here we propose a methodology for accurate local temperature measurement and subsequent specific absorption rate (SAR) retrieval using microscale thermocouples (TC). SAR was retrieved by fitting the measured initial temperature rise to the numerical solution of an equivalent thermal model. It was found that the accuracy of temperature measurement depends on thermosensor size, that is, the smaller TC, the more accurate the temperature measurement. SAR determined using TC with lead diameters of 25 and 75 μm demonstrated 98.5% and 80.4% match with computed SAR, respectively. However, both TC provided the same temperature rises in long run (> 10 min). FO probe failed to measure adequately local heating both for short and long exposures due to the relatively large size of the probe sensor (400 μm) and time constant (0.6 s). Calculated SAR in the cell monolayer was almost two times lower than that in the surrounding liquid. It was shown that the impact of the cell monolayer on heating due to its small thickness (5 to 10 μm) can be considered as negligible. Moreover, we demonstrated the possibility of accurate measurement of MMW-induced thermal pulses (up to 10 °C) using 25 μm TC. Bioelectromagnetics. 38:11-21, 2017. © 2016 Wiley Periodicals, Inc.

Millimeter‐Wave Heating in In Vitro Studies: Effect of Convection in Continuous and Pulse‐Modulated Regimes

“…The effect of a cell monolayer was numerically investigated in detail at 42 and 58.4 GHz for identical exposure conditions in Alekseev et al [] and Zhadobov et al []. They showed that the presence of a 10‐µm‐thick monolayer resulted in a 1% reduction of the total power absorbed in the sample and only slightly impacted the SAR distribution in the culture medium.…”

“…To experimentally validate the numerical results, the following temperature‐based approach was used to determine SAR [Zhadobov et al, ].…”

“…Among the methods used for thermal dosimetry at MMW frequencies (infrared thermometry [Chahat et al, ], thermo‐sensitive liquid crystals [Kojima et al, ], and fiber optic thermometry [Zhadobov et al, ]), microscale TC allows for accurate measurement of the local temperature with a time constant of the order of µs. This technique is particularly well‐adapted to measure the sharp MMW‐induced thermal pulses [Zhadobov et al, ].…”

“…At MMW frequencies, temperature measurements at the cell monolayer level are extremely challenging due to very local power absorption. A methodology to measure the temperature rise during in vitro exposure in the MMW band was reported in Zhadobov et al []. It was shown that a micro‐thermocouple (TC) could be used to measure the local continuous‐wave (CW) and pulsed‐wave (PW) MMW‐induced heating.…”

“…The initiation of convection in liquids by MMW and MW exposures has been described in several studies [Sharov et al, ; Khizhnyak and Ziskin, ; Betskii et al, ; Cueille et al, ; Paffi et al, ; Zhadobov et al, ]. Convection during in vitro bioelectromagnetic experiments affects the temperature dynamics and distribution [Sandblom and Theander, ; Pickard et al, ], causing the formation of a toroidal vortex [Khizhnyak and Ziskin, ] and temperature drop in the irradiated spot [Wessapan and Rattanadecho, ; Zhadobov et al, ]. Additionally, liquid motion caused by convective currents may perturb the local concentration of oxygen and nutrients near the surface of the cells [Paffi et al, ].…”